Stone pumpkins numbering in the hundreds, baked beige and reticulated by exposure to the summer desert sun, lie in a patch of several acres at the Imperial County site known as Pumpkin Patch. The spherical or oblate-shaped "pumpkins" are actually sandstone concretions, a somewhat mysterious type of durable sedimentary rock. Concretions can be found in many places throughout the Anza-Borrego Desert region and are known to assume a variety of sometimes intricate but always rounded shapes.

Daytime temperatures are dropping fast in the desert, down from the 90s of October to the 80s of early November. It's a good time, a fairly quiet time in fact, to make the trek out to the patch for a look at these natural curiosities. The trek consists of a two-hour-plus drive east from San Diego through Borrego Springs and toward the Salton Sea. You're going to need a four-wheel-drive vehicle, or at least a mini-SUV having more clearance and traction than the average sedan.

Turn southeast from Borrego-Salton Seaway (Highway S-22) at mile 34.9 onto the dirt road leading through Arroyo Salado Campground and down along the Arroyo Salado wash. Pass through the campground and continue 3.5 miles (from pavement) to the turnoff on the right for Seventeen Palms oasis (itself a worthy stop along the way). Our route to Pumpkin Patch continues southeast, curling up out of the wash and twisting over a low divide and past Five Palms. When you arrive at the next road junction, Tule Wash, stay left and follow the wide, sandy bottom of Tule Wash all the way to Pumpkin Patch on the right -- a short mile east of the San Diego-Imperial County line. At the county line, signs announce that you are leaving Anza-Borrego Desert State Park and entering Ocotillo Wells State Vehicular Recreation Area.

There they are before you, concretions littering the ground and coating the low hills to the south. Concretions, it is surmised, form in wet sediment that consists of fine, sandy particles. In this watery environment, there's a tendency for particles of like composition to accumulate in onion-skin layers around some "nucleus," or common point of origin. A cementing agent dissolved in the water -- silica, calcite, or iron oxide -- is necessary to bind the sand particles together. After drying, a bit of pressure perhaps, and thousands to millions of years, the durable concretions weather out of the softer sedimentary rock they're encased in and reveal themselves to the world above. The Pumpkin Patch concretions originated in the sediments of the ancestral Colorado River delta some three million years ago.